Report of Investigations 8176 Flame and Pressure Development of Large-Scale CH4-Air-N2 Explosions Buoyancy Effects and Venting Requirements By M. J. Sapko, A. L. Furno, and J. M. Kuchta Pittsburgh Mining and Safety Research Center, Pittsburgh, Pa. UNITED STATES DEPARTMENT OF THE INTERIOR Thomas S. Kleppe, Secretary BUREAU OF MINES Thomas V. Falkie, Director This publication has been cataloged as follows: Sapko, Michael J Flame and pressure development of large-scale CH4-airN2 explosions. Buoyancy effects and venting requirements, by M. J. Sapko, A. L. Furno, and J. M. Kuchta. [Washington] U.S. Bureau of Mines [1976] 32 p. illus., tables. (U.S. Bureau of Mines. Report of investi gations 8176) 1. Mine explosions. 2. Methane. I. Furno, Aldo L., jt. auth. II. Kuchta, Joseph M., jt. auth. III. U.S. Bureau of Mines. IV. Title. (Series) TN23.U7 no. 8176 622.06173 U.S. Dept. of the Int. Library Venting requirements for stoichiometric mixtures. Appendix A.--Derivation of venting equations 3 3 3 3 7 10 14 17 21 23 ... • ILLUSTRATIONS 1. Pressure histories for combustion of near-stoichiometric CH4-air mixtures with added N2 in a 12-foot-diameter sphere (central ignition at 300° K and 1 atm).. 2. Log-log plot of combustion pressure histories for nearstoichiometric CH4 -air mixtures with added N2 in a 12-footdiameter sphere.... 3. Side view of flame profile history for central ignition of a 9.4 4 6 8 5. 4. Variation of fireball radius with time for near-stoichiometric 8 9 6. Side view of flame profile history for central ignition of a 6.9 9 7. Burning velocity and vertical flame speed versus added N2 concentration for combustion of CH4 -air-N2 mixtures in a 12-footdiameter sphere (300° K and 1 atm).. 8. Burning velocity and vertical flame speed versus CH4 concentration for combustion of CH4 -air-N2 mixtures in a 12-foot-diameter sphere (300° K and 1 atm) 9. Burning velocity versus CH4 concentration for constant volume combustion of CH4 -air and CH4 -air-N2 mixtures (300° K and 1 atm).... 10. Buoyant velocity (v) versus square root of vertical fireball 11. ... radius (R) for constant volume combustion of near-stoichiometric CH4 -air mixtures with added N2. Critical fireball radius (vertical) for flattening versus vertical flame speed for constant volume combustion of CH4 -air and CH4 -airNa mixtures.. 12 12 13 15 16 ILLUSTRATIONS --Continued Page 12. 13. Vent area versus vessel volume for venting stoichiometric CH4 -air 18 21 Comparison of experimental and theoretical pressure rises versus A/v2/3 for 5.1 pct C3H-air ignitions in cubical and cylindrical chambers. (Experimental data obtained with unrestricted vents.). 19 14. Venting nomograph for spherical flame propagations in various size enclosures at 300° K and 1 atm (metric units)... Pressure rise versus vent ratio (A/v2/3) for venting stoichiometric CH4 -air ignitions with 0, 10, 20, and 30 pct added N2. lations by equation 10 for mixtures initially at 300° K and 1 atm.).. 15. (Calcu 22 TABLES 1. Empirical expressions for pressure development in combustion of various CH4 -air-N2 mixtures at 25° C in a 12-foot-diameter sphere Empirical expressions for flame speeds and buoyant velocities in combustion of various CH4 -air-N2 and CH4 -air mixtures at 25° C in a 12-foot-diameter sphere.. 2. 3. 4. Thermodynamic combustion properties of CH4 -air-N2 and CH4 -air mixtures at constant pressure (1 atm) and adiabatic conditions... Comparison of experimental and calculated venting data for gaseous explosions in various size enclosures... 5 11 14 20 The pressure and flame development of the methane-air-nitrogen system was investigated in the Bureau's 12-foot-diameter sphere to define buoyancy effects and to develop relationships for predicting venting requirements. Pressure rise (AP) is defined as a function of time (t) and nitrogen dilution and is shown to deviate noticeably from the cubic law (AP = kt3), depending upon the period of combustion and the mixture composition. Corresponding flame speed (S,) and burning velocity (Su) data are given which show that S, is time dependent and that N2 dilution has a greater effect than air dilution in inhibiting the flame propagation. The S data agree with "accepted" values and are unique in that they are based on much larger flames than those used in other investigations. Downward propagation limits are shown to depend mainly upon buoyant forces, which can affect the burning of even stoichiometric CH4air systems. Theoretical expressions, based on the mixture burning velocity and expansion ratio, are given for predicting the venting requirements for ignitions of hydrocarbon-air-nitrogen mixtures in spherical or cubical enclosures. INTRODUCTION 3 The behavior of large expanding flames is frequently difficult to predict and is necessarily of great concern wherever large volumes of flammable gases or vapors may be accidentally ignited. Buoyant forces are often cited to explain unusually high propagation rates, but sufficient data are not available to define the buoyancy effect as a function of fireball dimensions and mixture composition. Data on both flame and pressure development are essential in predicting explosion venting requirements and are particularly meager for partially inerted fuel-oxidant systems. Such information is presently needed by the automobile shredder industry and other industries for protection against ignitions of hydrocarbon vapor-air-inert systems. Supervisory research chemist. 1Chemical engineer. 3 Strehlow, R. A. Unconfined Vapor Cloud Explosions --An Overview. 14th Symp. (Internat.) on Combustion, University Park, Pa., Aug. 20-25, 1972, p. 1187. |